Ca²⁺ signalling in K562 human erythroleukaemia cells: effect of dimethyl sulphoxide and role of G-proteins in thrombin- and thromboxane A₂-activated pathways

The human leukaemic cell line K562 is a pluripotent stem cell with the potential to mature along a megakaryocytic or erythroid line. In these cells, thrombin and U46619 (9,11-dideoxy-9 alpha, 11 alpha-methanoepoxy prostaglandin F2 alpha), a thromboxane A2 analogue, increased intracellular Ca2+ in a rapid and concentration-dependent manner. The peak transient observed with both thrombin and U46619 was preserved upon stimulation in the absence of extracellular calcium and blunted with phorbol myristate acetate, suggestive of activation of phospholipase C. Short-term treatment with leupeptin abolished the calcium response to thrombin, but did not alter that to U46619. Both pertussis toxin (PT) and DMSO pretreatment inhibited thrombin- but not U46619-stimulated intracellular calcium elevation, indicating that these agonists signal through different G-proteins. Western blot analysis of crude membranes from K562 cells revealed the presence of G12 alpha and G13 alpha; the other known PT-substrates, Gi1 alpha and G0 alpha, were not detected. Consistent with this observation, ADP-ribosylation experiments revealed the presence of two PT substrates which co-migrated with human erythrocyte G12 alpha and G13 alpha. An antibody raised against Gq/11 alpha, a subfamily of G-protein alpha subunits unmodified by PT, specifically recognized 42 kDa protein(s) in K562 cells. PCR amplification of reverse-transcribed K562 RNA followed by DNA sequencing showed that these cells express messages for both Gq alpha and G11 alpha. Treatment of K562 cells with DMSO reduced the levels of thrombin receptor mRNA, without simultaneous changes in the expression of G12 alpha and G13 alpha. We have thus identified Ca(2+)-mobilizing agonists and related G-proteins in K562 cells, together with changes induced by DMSO in this signalling pathway.